As an internal combustion engine, a bi-fuel internal combustion engine for which a liquid fuel such as gasoline and a gas fuel such as CNG (compressed natural gas) can be used is known. In the bi-fuel internal combustion engine, a liquid fuel is used to start the engine, and thereafter, the fuel to be used for engine operation is switched from the liquid fuel to a gas fuel. A vehicle equipped with the bi-fuel internal combustion engine includes a storage container for storing a gas fuel. In the storage container, a gas fuel kept at a high pressure is stored. However, the gas fuel in the storage container contains impurities such as misty oil mixed in the process of compression to high pressure.
When the gas fuel containing oil is injected from an injection valve, oil adheres to the valving element and the valve seat, etc., forming the injection valve. When the outside air temperature reaches “−30° C.” or lower, the oil adhering to the injection valve is solidified and the injection valve does not normally open. Therefore, when the injection valve binds or may bind, it is preferable that switching from a liquid fuel to a gas fuel is prohibited.
Patent Document 1 discloses a method for judging whether or not the injection valve that injects a gas fuel binds. With this method, based on an outside air temperature, whether or not an environment in which an internal combustion engine is installed is at an extremely low temperature is judged. The result of use of this judging method for a bi-fuel internal combustion engine in which a fuel to be used during engine operation is switched from a liquid fuel to a gas fuel is as follows. When it is judged that the installation environment is not at an extremely low temperature based on an outside air temperature, it is estimated that the injection valve does not bind and the gas fuel can be normally injected from the injection valve. In this case, the fuel to be used for engine operation is switched from the liquid fuel to the gas fuel. On the other hand, when it is judged that the installation environment is at an extremely low temperature, it is estimated that there is a possibility that the injection valve binds and the gas fuel cannot be normally injected from the injection valve. In this case, the engine operation using the liquid fuel is continued.
However, the above-described judging method is based on the outside air temperature, and even in some cases where it is estimated that the injection valve does not bind, in actuality the injection valve still binds. If the fuel is switched from the liquid fuel to the gas fuel while the injection valve binds, a proper amount of the gas fuel corresponding to a required output is not supplied to a cylinder corresponding to the injection valve that binds. Specifically, smooth switching from the liquid fuel to the gas fuel cannot be performed.